Lithium-ion batteries (LIBs) have been widely utilized in various applications especially consumer electronics such as laptop computers, mobile phones, and digital cameras, etc. Recently lithium-ion batteries have started to be used in automobiles due to their superior energy and power density.
LIBs generally include an anode, a cathode, a separator, and an electrolyte. The anode and cathode are separated from one another by a separator in order to prevent short circuit while maintaining ion conductivity.
A separator is conventionally a thin, porous, electrically insulating material having high ion permeability, good mechanical strength and long-term stability to the chemicals and solvents used in the system, for example electrolyte of the electrochemical cell.
Separator for use in high performance battery system must be safe since very large quantities of energy are stored in the fully charged state in the battery. These energies must not be released in an uncontrolled manner in the event of malfunctioning of the battery, such as overcharging or short-circuit, since this would lead to an explosion or ignition of the battery.
Generally, a typical organic separator consists of a composite film comprising a polyolefin-based substrate and an inorganic coating layer. A major disadvantage of these polyolefin-based separators is their low thermal stability limit. When the battery temperature exceeds 150° C. or lower, the organic separator rapidly shrinks. A cathode and an anode will directly contact to each other, to cause enlargement of short-circuited area. Therefore, such a separator is prone to cause battery short circuit and is generally not safe.
As a result, it is necessary to provide a separator that does not cause heat shrinking at high temperature.
JP Pat. No. 5617609 has disclosed a separator having a shutdown function. The separator comprises an ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE) and an inorganic oxide, and has a shutdown temperature of approximately 134° C. which is the melting point of the HDPE. However, because such separator is still a polyolefin-base material, they have a disadvantage of obtaining a significant improvement in safety including prevention of heat shrinking at high temperature. Therefore, there is a great possibility of short-circuit between a cathode and an anode caused by shrinking or melting of separators in the event of an accident.
In another aspect, a separator must be strong enough to withstand the tension of the winding operation during battery assembly. In order to prevent possibility of damage such as a tear of the separator during a manufacturing process, different attempts have been made to solve the problem and improve the performance of the separator including mechanical strength, abrasion resistance and flexibility, etc.
U.S. Patent Application No. 20140212727 A1 has described a separator having a good mechanical stability for the ease of handling in battery manufacture. The separator comprises fibers of an electrically nonconductive material, and a porous electrically nonconductive coating comprising oxide particles, an inorganic adhesive and polymer particles. The separator shows improved laminatability in contact with electrodes, improved flexibility, higher tolerance to buckling, and greater penetration strength. However, the inorganic adhesive has been prepared via a particulate sol or a polymeric sol. This makes the manufacturing process more complex. In addition, only some physical properties, such as MacMullin number and Gurley Number, of the separator have been measured. However, the physical properties of the separator alone do not mean superior battery performance.
U.S. Patent Application No. 20150380705 A1 has described a polyolefin separator which has high tensile strength and low melt shrinkage in the machine direction (MD) and the transverse direction (TD) through regulation of a stretching process. This involves subjecting the separator to TD relaxation to remove stress from the separator subjected to the TD stretching. However, regulation of a stretching process is not suitable for separator that operates at high speed on a roll-to-roll process. Furthermore, the separator made of polyolefin will easily shrink even at a temperature of about 150° C. or lower.
In view of the above, there is always a need to develop separators which have distinctly improved mechanical and thermal stability and are suitable for mass production to meet the performance requirement in lithium high power batteries.